The present invention relates to compressors that are employed in automotive air-conditioning systems.
Variable displacement compressors are often used in automotive air-conditioning systems. A typical variable displacement compressor has a crank chamber, which is defined in a housing. A drive shaft is rotatably supported in the crank chamber. The housing includes a cylinder block through which cylinder bores extend. A piston is accommodated in each cylinder bore. A cam plate is fitted to the drive shaft and arranged in the crank chamber. The cam plate is supported such that it can be inclined while rotating integrally with the drive shaft. Each piston is coupled to the cam plate such that the rotation of the drive shaft reciprocates the piston and compresses refrigerant gas. The compressed gas is then sent into a discharge pressure zone, which is defined in the compressor housing. Afterward, the gas is discharged from the compressor to circulate through an external refrigerant circuit. The gas then returns to the compressor and enters a suction pressure zone, which is also defined in the compressor housing. Lubricating oil is suspended in the refrigerant gas. Thus, the refrigerant gas functions to lubricate moving parts. The displacement of the compressor is controlled by adjusting the amount of refrigerant gas drawn into the crank chamber.
The discharge pressure zone and the crank chamber are connected to each other by a pressurizing passage. The crank chamber and the suction pressure zone are connected to each other by a bleeding passage. A displacement control valve is arranged in the pressurizing passage. The displacement control valve adjusts the opening size of the pressurizing passage to restrict the amount of refrigerant gas passing therethrough in accordance with the pressure of the suction pressure zone. This controls the amount of refrigerant gas that is sent from the discharge pressure zone to the crank chamber and alters the pressure of the crank chamber. The difference between the pressure in the crank chamber, which is applied to one side of the pistons, and the pressure in the cylinder bores, which is applied to the other side of the pistons, causes the cam plate to incline with respect to the drive shaft. This changes the stroke of each piston and varies the compressor displacement.
The moving parts in the crank chamber are lubricated by lubricating oil residing in the crank chamber. Refrigerant gas leaks between each cylinder bore and the associated piston and enters the crank chamber. The gas leakage, or blowby gas, contains a large amount of lubricating oil. Thus, the amount of lubricating oil residing in the crank chamber depends on the amount of blowby gas. When the compressor is operated with a high displacement, the compression ratio of the refrigerant gas increases. This, in turn, increases the amount of blowby gas. Accordingly, the crank chamber is supplied with a sufficient amount of lubricating oil.
However, when the compressor is operated with a low displacement, the compression ratio of the refrigerant gas decreases. This, in turn, decreases the amount of blowby gas. In addition, the lubricating oil residing in the crank chamber is agitated and thus atomized by rotating parts such as the cam plate. The atomized oil is mixed with the refrigerant gas and forced toward the suction pressure zone through the bleeding passage. Therefore, the amount of lubricating oil with which the crank chamber is supplied may become insufficient, especially, when the compressor is of a variable displacement type that increases the pressure of the compressor by sending refrigerant gas into the crank chamber from the discharge pressure zone. Accordingly, the decreased amount of lubricating oil in the crank chamber may result in insufficient lubrication of the moving parts.
In a fixed displacement type compressor, the refrigerant gas that returns to the compressor from the external refrigerant circuit typically flows through the crank chamber before entering the suction chamber. Thus, the lubricating oil in the crank chamber has a tendency to escape into the suction chamber. Accordingly, the crank chamber must constantly be replenished with lubricating oil.